1. Related Application
This application is related in subject matter to commonly owned application Ser. No. 07/326,581 filed Mar. 21, 1989 by the same inventors as herein entitled "Blow Molding Machine For The Blow Molding Of Hollow Articles With Waiting Time Control."
2. Field of The Invention
The invention relates to a blow molding machine for the blow molding of hollow articles, with an accumulator head, which is connected to an extruder supplying synthetic material such as plastic, and from which in the filled condition a discharge piston drives the plastic through an annular slit-shaped discharge opening to produce a preform, with a blow mold for forming a hollow article from the preform and with a control system which, after completion of a blow molding sequence, derives a piston positioning signal for regulating the position of the discharge piston from a comparative signal, which is formed from the difference between the actual position and the set position of the discharge piston, the set position being determined by integration from a previously specified speed set point profile.
3. Description of the Prior Art
In order to blow mold hollow articles from thermoplastic synthetic material, tubular shaped preforms are produced from the relevant plastic material using an accumulator head and in a further operation are inflated between two hollow mold halves (blow mold) to the final size. The accumulator head is connected to an extruder, from which the thermoplastic synthetic material reaches the accumulator head. The accumulator head has an annular slit-shaped discharge opening, which is adjustable. The thermoplastic material is driven out of the discharge opening by a discharge piston. The preform which results in this way is stretched nonuniformly depending on the shape of the hollow article to be obtained. Thus, a uniform wall thickness of the hollow article requires a profiling of the wall thickness in the preform (wall thickness set point profile). This wall thickness set point profile of the preform is produced by adjusting the slit width of the discharge opening during the discharge step and altering the speed of the discharge piston in the discharge stroke. A blow molding machine working in this manner is known from German Offenlegungsschrift No. 3,416,781.
During a discharge step the speed of the discharge piston is determined by regulating the position of the discharge piston. A piston actuator displaces the discharge piston depending on the piston positioning signal of a position regulator. The position regulator receives a comparative signal, which is formed in a superimposition stage by subtraction of the actual position from the set position. Here, the set position is determined by integration from a previously specified speed set point profile. The speed set point profile is divided into a certain number of segments, which define various positions. An alteration in the speed may occur from one segment to the next. This speed curve which has been preselected by the profile corresponds to the profile for the wall thickness control at the discharge opening.
The speed set point profile is determined for a certain volume within the accumulator head. If the volume of plastic within the accumulator head remains constant in successive discharge steps, a preform is produced (reference preform) which essentially exhibits the same features, namely constant length and constant volume. Here, it is necessary that the influx of plastic from the extruder remains constant. If this influx of plastic varies, e.g. by alteration of the rotational speed of the extruder, the discharged tube also varies.
Normally the discharge step begins when the accumulator head is filled with thermoplastic material (until the set point contents level is reached) and the blow molding sequence for producing a hollow article is complete. If the blow molding sequence is not yet finished, the accumulator head continues to be filled with plastic, until after the conclusion of the blow molding sequence the discharge step can begin. The additional influx of plastic caused by the overfilling of the accumulator head causes variations in the preform. Profiling of the preform is only undertaken when the additional plastic has been discharged as a so-called "run-in tube". A waiting time also arises when the blow molding sequence is complete, but the accumulator head is not yet completely filled to its set point contents level. A loss of material and a reduction in the number of hollow articles which can be produced per unit time (reduction in capacity) results from the waiting time between completion of the blow molding sequence and reaching the set point contents level in the accumulator head and vice versa.